1. Field of the Invention
The present invention relates to a shape measuring apparatus and a shape measuring method and, more particularly, to a shape measuring apparatus and a shape measuring method suited to accurately measure an aspherical shape, which is difficult to measure by a general interferometer, among other shapes of smoothly continuous objects such as lenses having comparatively large diameters, mirrors, and metal molds used in the fabrication of semiconductor devices.
2. Related Background Art
Conventionally, many interference apparatuses using interference phenomena of light are used to measure object surface shapes such as an aspherical shape. FIG. 12 is a schematic view showing the main components of a surface shape measuring apparatus disclosed in "Optics", Vol. 12, No. 6 (December 1983), pp. 450-454.
Referring to FIG. 12, this apparatus includes a Zeeman laser 901 as a light source, a beam splitter 902, polarization beam splitters 903 and 904, .lambda./4 plates 905a and 905b, an objective lens 906, a reference surface 907, a work (object to be measured) 908, a work stage 909, a focus detector 910, and beat signal detectors 911a and 911b.
In FIG. 12, two light components f1 and f2 emitted from the Zeeman laser 901 as a light source, polarized in directions perpendicular to each other, and having slightly different frequencies f1 and f2 are split into two parts by the beam splitter 902. Light components passing through the beam splitter 902 are spatially separated by the first polarization beam splitter 903.
Of these light components, the light component f1 travels straight, passes through the second polarization beam splitter 904, and is converted into circularly polarized light by the .lambda./4 plate 905a. This light so strikes as to be focused on the surface of the object (work) 908 by the objective lens 906. The light is returned to the objective lens 906 by so-called cat's eye reflection and converted into linearly polarized light by passing again through the .lambda./4 plate 905a. This linearly polarized light enters the second polarization beam splitter 904 such that the polarizing direction of the light is rotated 90.degree. from that of the linearly polarized light before the reflection.
A special coating is formed on this polarization beam splitter 904. Accordingly, the polarization beam splitter 904 splits the returned light into two parts, transmits one part to the first polarization beam splitter 903, and reflects the other to the focus detector 910.
By using a signal from the focus detector 910, the objective lens 906 is moved by servo control in an optical axis direction indicated by the arrows such that the light is always focused on the surface of the work even if the work moves in a direction perpendicular to the optical axis.
On the other hand, the light component f2 reflected by the polarization beam splitter 903 is converted into circularly polarized light by the .lambda./4 plate 905b. This circularly polarized light is reflected by the reference surface 907 arranged on the work stage through the lens and the mirrors and returned to the polarization beam splitter 903. Since the light passes again through the .lambda./4 plate 905b, this light is converted into linearly polarized light whose polarizing direction is rotated 90.degree.. Therefore, the light propagates to the beat signal detector 911b through the polarization beam splitter 903.
The light reflected by the work 908 and returned to the polarization beam splitter 903 also propagates to the beat signal detector 911b. Therefore, this light interferes with the light reflected by the reference surface 907, and the beat signal detector 911b detects a measured beat signal (F1-F2).
The light components f1 and f2 immediately after being emitted from the light source 901 are reflected by the beam splitter 902 and caused to interfere with each other to obtain a reference beat signal by the beat signal detector 911a. The phase difference between the measured beat signal obtained by the beat signal detector 911b and the reference beat signal obtained by the beat signal detector 911a is measured. This phase difference is integrated by a phase difference when the work 908 is scanned in the direction perpendicular to the optical axis. In this manner, the surface shape of the work 908 is measured.
The surface shape measuring apparatus shown in FIG. 12 obtains wave surface information of reflected light of light focused on the surface of the object 908. In this method, if a small dust particle or flaw is present on the object 908, the reflected light is scattered. This extremely changes the amount and phase of light returning to the detector 911b and makes the measurement difficult to perform. Consequently, an integrating counter error occurs to interrupt the measurement at that point.
Another conventional shape measuring apparatus is shown in FIGS. 13 and 14. FIG. 13 is a schematic view showing the major parts of a three-dimensional shape measuring apparatus proposed in Japanese Patent Publication No. 2-11084. FIG. 14 is a view for explaining a part of FIG. 13.
Referring to FIG. 13, two light components f1 and f2 emitted from a Zeeman laser 601 as a light source, polarized in directions perpendicular to each other, and having slightly different frequencies are partially guided to a photodetector 604 by a beam splitter 603. The photodetector 604 detects a reference beat signal. The light components passing through the beam splitter 603 enter a first polarization beam splitter 605. Of these light components, the light component f2 is reflected upward, condensed by a lens, and reflected by a fixed mirror 607 to reach a photodetector 608 through a lens and the polarization beam splitter 605.
The other light component f1 travels straight through the polarization beam splitter 605, passes through an objective lens 613 through a half mirror HM, and reaches the surface of a work (object to be measured) 609. The light is reflected by the surface and returned to the first polarization beam splitter 605 through the forward optical path. The light is then reflected by the first polarization beam splitter 605 to reach a photodetector 608 and interfere with the light f2. Consequently, a measured beat signal is detected.
By integrating a frequency difference .delta.f between the reference beat signal obtained by the photodetector 604 and the measured beat signal obtained by the photodetector 608, an optical path length change of the light components f1 and f2 is measured. That is, a displacement (shape) in the optical axis direction of the work (object) 609 is measured.
In the surface shape measurement shown in FIG. 13, the measured beat signal cannot be detected unless light is always focused on the surface of the work 609. In this apparatus, therefore, a part of the reflected light from the work 609 is guided to photodetectors 611 and 612 via the half mirror HM. The positions of the objective lens 613 in an optical axis direction (Z) and a direction (X) perpendicular to the optical axis are servo-controlled such that light is always incident in the normal direction of the work 609 and kept focused on the surface of the work 609.
In this state, the work 609 is rotated (.theta.) about its axis of rotational symmetry by a driving means 623 and at the same time moved in a radial (X) direction. In this manner, a position where the light is incident is scanned in a cylindrical coordinate system (X-.theta.-Z system) to measure the entire surface shape of the work 609.
Additionally, as shown in FIG. 14, a work rotation axis 624 is inclined by an angle .beta. in the X and Y planes to measure the work by using the full-aperture angle of the objective lens 613. This enables the measurement of a work having a large plane inclination.
In the measuring apparatus shown in FIG. 13, however, measurement errors of the surface shape of the work 609 are caused by, e.g., rotation errors of the work rotating stage and position read errors of the work radial direction moving stage. Accordingly, the measurement accuracy limit is dominated by the mechanical kinetic accuracy.
Furthermore, the embodiment by which a large inclination angle is measured by inclining the work rotation axis 624 to the radial direction movement axis has the following problems.
(A-1) Work support deformation is increased by oblique application of gravity.
(A-2) A large space in the apparatus is occupied by a work, and this increases the size of the apparatus.
(A-3) When a system for measuring and correcting the kinetic accuracy of the rotating stage is added, measurements based on external standards become difficult to perform because the rotating stage involves radial movement.
(A-4) It is difficult to automatically attach and detach a work.